Some three million of those users were lost because of a change to the Shared Links feature in the mobile Safari browser, which displays links from people's Twitter followers. The other million were "Twitter-owned," CFO Anthony Noto said during Twitter's earnings call, without explaining further.

The vague remarks confused some people on Twitter's quarterly earnings call. "Makes no sense to me," tweeted Mark Gurman, senior editor at Apple news site 9to5Mac.

CEO Dick Costolo told Business Insider later that the one million users it lost couldn't get their passwords to work because of an encryption issue related to Twitter's integration with iOS.

Four million isn't a huge number to Twitter, which had 288 million users at the end of last quarter. But even without the Apple glitch, Twitter is struggling to maintain its historic growth. It had 20 percent more users last quarter than at the end of 2013, the lowest growth rate it's yet reported.

On its earnings call, Costolo referred to the issue as an "unforeseen bug in iOS 8 as it relates to Twitter integration into the OS." He added that Twitter has a "great relationship with Apple."